Method of sealing dissimilar materials



Feb. 10, 1942. H. K. KING METHOD` OF SEALING DISSIMILAR MATERIALS Filed March 18, 1941 INVENTOR TT'ORNEY HKENDALL KING Pateted Feb. '10, 1942 UNITED STATES PATENT OFFICE METHOD OF SEALING nssmmt MATERIALS Henry K. King, Cabin John, Md.

Application March 18, 1941, Serial No. %3,943

(Granted under the act of March 3, 1883, as t amended April 30, 1928; 370 O. G. 757) 17 Claims.

This invention relates to the method of uniting dissimilar materials, and more particularly to the method of securing metal to porcelain or other ceramic material to form a pressure resistant gas tight seal. Thisinvention opens up a new technique in the construction or manufacture of electrical appliances requiring a metallic conductive member to be sealed .to an insulating carrier member. In many industrial products it is desirable to unite such dissimilar materials so that they may be held together strongly and with their surfaces in such intimate contact as to prevent the formation of t voids allowing foreign material to accumulate therein, and also to prevent leakage of gas at the surface of contact.

In other uses such joints must permit good thermal transmission from the surface of one material to the surface of the other. Since this method is particularly useful in the construction of spark plugs, producing a product having advantages heretofore unattainable, I have described and illustrated as specific examples of my improved method the Construction of several types of spark plugs, as well as the construction I of a high voltage insulator, although it will be understood that many other articles may be united in construction by the process hereinafter defined.

Since my invention is particularly adapted for -use in the Construction or assembly of spark plugs, and since I have selected this specific embodiment 'as *illustrative of my improved method, I shall. discuss some of the advantages 'it ofiers in this art, heretofore practically unattainable. In the Construction and assembly of spark plugs considerable difliculties have been encountered in securing the porcelain insulator to the outer metallic jacket of the plug so as to obtain a good gas-tight seal, since the outer metalli jacket must be clamped against the porcelain or other refractory insulator with sufncient pressure to insure a good gas-tight seal of the products of combustion at the pressures usually encountered in a high compression aircraft engine and under the conditions to which the plugs of such engines must be subjected.

In obtaining this seal by the prior art methods there is a great possibility that the` porcelai'n insulator will crack under clamping process, whereby its insulatingproperties would be destroyedmThe patent to Warner, 1,498,858, is illustrative of one of the methods proposed to obviate this possibility in construction. The

method proposed by Warner has several inherent' disadvantages which are not present in the spark plugs constructed under the method to' be described hereinafter. For example, in the type of Construction illustrated, the forces applied indirectly to the porcelain through the metalllc colconsequently produce a resultant force setting up tensional stress in the insulator. Ceramic insulators are strong under compression but not under tension, therefore the insulator may crack. Furthermore, the metal and the porcelain have, in general, radically different coemcients of expansion and the metal collar will tend to 'expand away from the porcelain upon being heated to its Operating temperature, .whereby the seal will be destroyed. It is further noted that the metal collar is clamped only at the conical surfaces, therefore the cylindrical portion of the collar will buckle or be otherwise distorted as *the collar expands. It should also be noted that this 'patentee has also failed to provide a satisfactory scaling means between the inner electrode and the porcelain insulator.

The spark plug con'structed under th method set forth in the U. S. Patent to Furber, 1,186,568, is impracticable since the surfaces of the jacket and insulator cannot, under this method, be brought into such intimate contact to form a gas-tight seal over a wide temperature range without cracking the porcelain insulator. When the insulator and metal are united under the' method setforth in the patent to Devers, 1,760,586, a more satisfaetory seal is obtained without subjecting the insulator to any severe stresses. However, the seal of the electrode, as

" illustrated, would be unsatisfactory in practice.

provide a method of securing dissimilar bodiea together so that the junction of the surfaces will form gas-tight seal which will remain gas# tightthroughout the range ot Operating temperatures, irrespective o! the fact that the mate- It is, therefore, an object of my invention to rials have radically different coemcients of .expansion.

It is a iurther object of my invention to provide an improved method of assembly or manufacture ot spark plugs by which a gas-tight seal between the insulator and the body is produced in which compressive stresses are alone set up in the insulator so that breakage of the insulator, due to sudden and severe changes in temperature, is substantially eliminated.

It is also an object of my invention to provide an improved method of joining the insulator to the outer metallic shell of' spark plugs, so that their respective surtaces are in such intimate contact that' the joint offers substantially no resistance to the transfer of heat !rom the insulator to the outer shell.

The further objects ot my invention reside in the provision of a simple, inexpensive method for joining dissimilar materials, and in the new and improved products which are formed thereby.

With these and other objects in view, as well as other advantages not herein set forth that may beincident to the use of my improved method or the products produced thereby, theinvention consists in combinations hereinafter claimed, with the understanding that several of the necessary steps constituting same may be varied 'within the scope of what is hereinafter deflned in the appended claims.

In order to make my invention more clearly understood, I have shown in the accompanying drawing, means for carrying this invention into practical'use, without limiting the application of said invention to the specific examples illustrated in the accompanyingdrawingfln which:

Fig. 1 is a longitudinal sectional view of a. cylindrical hollow porcelain insulator oi a spark plug which is to be assembled in accordance with my 4 invention;

Fig. 2 is the same view of Fig. 1 after the material of Fig. l has been subjected to the first method step;

Fig. 3 is the same view oi' the materials of Fig. 2 as they appear ata later step in the process;

Fig. 4 is a longitudinal sectional view of the spark plug showing the insulator secured to the outer metallic jacket, in accordance with the invention:

Fig. 5 is a longitudinal sectional view of" the finished product made by the method of my invention;

Fig. 6 is a longitudinal view of a shielded spark plug constructed in accordance with my invention;

vention as applied to mounting a metallic support.

in a high voltage insulator;

Fig. 9 is an exploded view illustrating a modification of the method embodying my invention as applied to the assembly of a spark plug illustrated in Figs. l to 3, inclusive; and

Fig. 10 is a longitudinai cross-sctional view of a further modification.

Reierring now to the drawing, particularly Figs. 1 to 5, inclusive, showing my invention as applied to the assmblyof a non-shielded spark plug whose component parts ar'e illustrated therein, and comprise the cylindrical ceramic insulator shown in Fig. 1, which is provided with a longitudinal bore I2 adapted to receive and securely hold the electrode rod IS; and an outer i a high temperature.

metallic jacket H into which the insulator ID is adapted to be inserted. My invention is carried out by the process which comprises first coating the hollow cylindrical insulator o on both its inner and outer surface over an area at which the seal is to be formed, with a suitable preparation containing the metal to be used iri the formation of the seal. The coated areas on the ceramic insulator, as indicated in Figs. 2 and 3 at I 4 and !5, are of an exaggerated thickness and may be applied to the insulator by any of a number of processes, all of which at present appear equally effective. For example, the area at which the seal is to be formed, as indicated in Fig. 2, may be painted with a suitable colloidal solution of gold, platinum, silver or other metal and the metal then fused into the insulator at Instead of the colloidal solution, the metal may be applied directly in the form of a foil or leaf and then fused into the insulator at a high temperature; or the metal may also be applied directly to the insulator by "sputtering," "cathode spraying," "silvering," or such other process well `known to the art. The method which I prefer to use is the colloidal solution method, in which the metal is fused onto the porcelain in much the same manner as it is in the art of applying gold or platinum decorations to china dinnerware. One important feature of my invention, as will be apparent presently, resides in the provision of the metal to both the inner and outer surfaces of the insulator.

. After the thin metal coating has been applied` by any of the processes outlined above, it is built up to thedesired thickness by electroplating, fusing or other process which includes "sputtering," metal spraying," silvering," or by apply- 'ing a metallic band to the inner and outer sur- Iaces by means of an expanded and shrink fit, respectively,` although I prefer to build up the metal to the desired thickness, as shown at Ha and |5a of Fig. 3, by means of an electro-plating process after the metal coating contained in the colloidal solution has been fused into place and allowed to cool. It may be desirable under some conditions to apply the first coat and also to build the metal up to its desired thickness by a single process such as sputtering" or spraying." consequently, in Figs. 4 to 10, inclusive, I have illustrated the metallic band 'as being built up of a single metal. I have done this primarily for the sake of clearness in the drawing; however, if the same metal were to be ap plied under the diflferent processes, it would be proper to indicate same by means of single hatching.

After the metal is built up to the desired thickness it is machined down to a precalculated inner and outer diameter, as shown in the dotted outline of Fig. 3. The insulator is now ready to be inserted into the metallic jacket II to form a gas-tight seal therewith. Before inserting the insulator and metallic bands into the metallic jacket' H, ,the inner diameter oi' the jacket is expanded by heating the jacket to a high temperature, and the diameter of the outer metallic band ll is reduced by cooling the insulator. I prefer to cool the insulator by inserting it in liquid air or in carbon dioxide Snow; although other processes may be used. The cooled insulator is then inserted into the heated metallic jacket and the assembly brought to a uniform tem perature. The entire unit is now heated' and the electrode rod H is cooled by the same process as applicd above, and the electrode rod 4 is inserted into the metalized bore l2. The entireunit of electrode rod |3, porcelain o, metallic rings !4 and !5, and outer,.metallic -jacket ||,.are now eectively sealed together fof all conditions to which the plug may be subjected, since they are now in intimate thermal contact 'and a sufcient temperature differential between these members cannot be produced to reverse the shrink fit process. i r

It should be understood that the step of cooling is not applied solely for the purpose of obtaining a maximum temperature differential which will never be attainablen practice, but also to place the insulated member under compression by causing the outer cylindrical ring M to be shrunk tightly about the insulator, so that when the insulator and jacket are both heated to provide for the insertion of the electrode rod !3, the metal band will not be expanded away from the insulator, and thereby destroy the effective seal between the band and insulator. This is one of the important distinguishing novel features of my invention, which has rendered satisfactory the products produced by this method, while the products produced under methods as outlined in the prior artflwith which I am familiar, have failed. e. e

Furthermore, the inner ring 15 also serves a double function in that as the insulator and jacket assembly shown in Fig. 4 are heated to allow for the insertion of the cooled electrode, which has a greater coefficient of expansion than the porcelain insulator, the inner ring is thus expanded into intimate compressive contact with the insulator, so that compressive stresses are set up therein, whereby the seal will be preserved after the assembly attains the final temperature.

The metal used in forming the rings M and |5 is preferably a soft metal such as copper, which,

under the compressive forces referred to above will be firmly pressed into the pores of the metallic jacket and the electrode and will be retained in its intimate contact with the porcelain by these compressive forces. The outer metallic ring !4 may also be thinly coated with a metal having a low melting point, such as tin or the like, so that as the Operating temperature of the jacket and ring i i is increased, this metal will become` molten and will be forced into whatever interstices that nay still exist between the band 4 and the jacket. The joint between the band and the jacket may also be spot welded at the top of the band since the band and the insulator are under tension and can thus' withstand the heat of the' weld. From tests to which spark plugs constructed under the method herein presented have been subjectd, it would appear that the provision of the coat of low melting point metal or the welds would beunnecessary. They may; however-be used `merely as van added protection against gas leakage past the seal. i

In actual construction 'the' coeicients of i expansion of all materials are'known and the temperatures to which all parts arefs'ubjected are precalc ulated, so that compressive forc'csst up by the bindin'g action of the inner and outer ring M, and IS, respectively, are substantially balanced over the normal Operating temperatures of the plug. Tocompletethe spark plug structure, a p rk o lS'js convenientlymounted on the jacket H and a suitable 'conductive button is threaded into the body of the insulator n; The

i binding nut Is is'jthreaded` ontoan end of the electrode rod so thatth i'gnition'ca'ble may be secured to the plug between the button and the binding nut to be in current conductive relationship with the electrode.

.In Fig. 6 I have illustrated a radioshielded spark plug constructed by the method outlined above, and in this Construction, in which similar reference numerals have been used to indicate similar parts, the outer jacket H' has a longitudinal extended portion which completely surrounds the refractory insulator o' and forms the shieldng element thcrefor. The shielding element is provided with a threaded portion to which the ignition cable cap !9 may be removably. secured. In this spark plug the shape and conguration of the insulator IO' is modified somewhat in accordance with the plug Construction, and a plurality of spark points IE' are mounted on the jacket. Other plug details are similar to those illustrated in Fig. 5.

Fig. 7 illustrates this invention as applied to a mica insulated spark plug, having a unitary shell member comprising a shielding element 2l shown as an extension of the threaded base 22, there being an intermediate flanged 'portion 23 having the usual polygonal section 24, which reccives the wrench or other tool for nserting the plug into the cylinder wall. The shielding portion is threaded to accommodate a cap similar to that shown in Fig. 6.

In the construction of this spark plug the hollow cylindrical refractory bushing 25 is provided with the metallic bands' 26 and 21 on its inner and outer surface by any combination of the methods set forth above, and the refractory bushing and bands are secured to the shell member 20 by the same process outlined above.

Furthermore, the electrode rod 28 is also inserted into the refractory insulator 25 by the process outlined above, Whereby a tight seal between jacket, insulator and electrode is obtained. Around the lower part of the electrode rod 28 a stack of mica washers 29 is disposed which rest on shoulder 30 formed on the electrode, so as to press the stack of washers between the bushing 25 and said shoulder. Above the bushing 25 and surrounding the electrode rod 28 is a second stack of mica washers 3I. The upper end is formed so as to compress the washers between the porcelain bushing and a Washer 32 positioned over v the upper end of the electrode.

In the past, considerable difficulty has been exservice, by reason of the fact that under high temperatures the end of the porcelain insulator will chip away from the metallic electrode at its base whichmay, in time, cause the plug to fail. This chipping of the insulator is particularly noticeable where the spark plugs are used on aviation engines. Furthermore, lead fouling is more pronounced in the porcelain type of plug, with the result that the plugs unless frequently cleaned may in time be shorted out. The plug which I have illustrated in Fig. '7 is designed to overcome these disadvantages and at the same time retain the advantages of the porcelain insulator with the improved scaling means herein disclosed. The mica is capable of withstanding the excessively high temperatures of combustion and the lead fouling is less pronounced with mica than with porcelain.

In Fig. 8 I have shown my invention as applied to the Construction of a high voltage pin type insulator. In the .prior art these insulators were usually attached to the iron pins by screw threads in which the threaded portion of the pin is tapered downwardly from the top of the pin. A

good t in the threads between the pin and the insulator is not commonly attained because the porcelain of the insulator warps in manuiacture so that the hole ceases to be circular or the sui'- face oi the thread may not be smooth. consequently. in attemptin'g to insert the pin, the insulators are frequently cracked. In order to avoid this the tread of the pin is usually made of lead or other soft metal, but even this is unsatisiactory, since the insulators become loose with changes'in temperature, due to the diii'erent coemcients oi' expansion. The construction by the method involved inmy invention has overcome many oithese disadvantages. i

The body oi' the insulator I is provided with a metallic tension band ll, iormed around the base of the insulator. This band ll may be placed on -in accordance with the process involved in my invention, although care must be exercised in placing this band on the insulator so that its expansion will not set up excessive ten sional stresses in the nsulator. It is preferable to provide a clearance between the outer surface of this band and the body of the insulator as illustrated in Fig. 8. In the process of assembly I first 'coat the inner surface of the pin hole 42 with a colloidal solution o! the metal which is to be used. This metal is then !used into the insulator at a high !using temperature, after which the metallic coated insulator is allowed to cool. I then proceed to build up the metal to' the desired thickness by electroplating or other equivalent process which I have set !orth above. The metal is then machined t the precalculated inner diameter. The outer tension ring ll is preferably applied after the insulator has been' heated, but may be built up under the process deflned above if suitable ciearance is pro-' vided to allow the band to expand without transmittlng excessive tensional stresses to the outer surface oi' the insulator. metallic bands are now heated to a precalc'ulated temperature to expand the inner diameter oi the metallic lined pin recess and the pin u is cooled down by being immersed in liquid air or carbon dioxide snow, and is inserted into the insulator,

which upon attaining final temperature produces a tight joint.

It should be again noted that the inner band is expanded into a binding relationship with the insulator so that compressive stresses are set up therein and the band !I also sets 'up compressive stresses in the opposite direction so as to prevent any breakage during the construction process,

and to equalize the forces o! the inner band.

In the modification shown in Fig. 9, a soft metallic band l! is applied directly to the metallic shell II, preierably by ashrink or expanded nt process so that 'the band is s at normal `tempcratures. The band and shell Il and Il are now heated toexp'and the inner' diameter and the coemcient oi' expansion o! the two metals are selected so that the Expulsion of the inner band is greater than the outer shelLso that a tight seal is at ali times maintained. The porcelin insulator 41 isnuw` cooled down by placed in liquid air or carbon dioxide snow and inserted into the xp nded metal shell. 4

4 The eleetrode rod II has a metallic band of soft metal u secured thei-eto by a shi-Ink process or any other applicable method, and this electroderodaterbeingcooledisinserted intothe 4 expanded insulator and shell ssembly. Upon attaining' the final temperature. the soft metal .ot .the-inner and outer band is forced into inti- The insulator and mate contact with the porcelain, formin: an exceedingly tight seal.

As is apparent from Figs. 1 to 8, inclusive. difliculties may be experienced in attemptlng to apply the metallic coating to the inner surface of the insulator because of the restricted bore ll, and in order to avoid such difliculties in construction I have devised and illustrated in Fig. 10 a modified form o! a shielded spark plug, constructed under the process involved in my invention, as set out above. in which the diameter of the bore !I of the insulator ll is materially increased so that the metallic coating il formed on the inner surface oi' the porcelain insulator may b easily applied and conflned to that area over which the sealis to be iormed. The in- -sulator sa is also provided with an outer metallic band ll which s applied by any of the processes outlined above.

The outer metallic shell 52 is heated to a predetermined temperature and the cooled insulator with bands ll and I! is inserted therein. The inner electrode 53 has an integrally iormed middle portion 54 having an increased diameter. This electrode is cooled down and inserted into the heat expanded metallie jacket, insulator, and bands so that the electrode will form a seal with the inner band at the outer surface of the cylindrical portion 51. The outer jacket is also provided with an ignition cable cap I! threadbly Secured to the metallic shell 52., The cable conductor is embedded in any suitable insulating material 56 and is connected to the electrode rod 53 by means ot a coiled resilient electrically conductive member 51. Itis thus apparent that the plug which I have disclosedwould be easier to t manufactre thanthe plug illustrated in Fig. B,

and still retain many of the advantages of this type of spari: plug.

Other modflcations and changes in" the numi'actured and/or`used by or for the Government of the United states of America !or governmental purposes without the p'ayment of any royalties thereon or therefor.

Having thus set forth and disclosed the nature oi this invention, what is claimed is:

1.' In the manufacture oi a spark plug, the method o! mounting and scaling the ceraxnic insulator in the outer metallic jacket o! the spark plug which comprises. the steps oi applying a thin coating of metal to the ceramic insulator so that it may be in intimate contact with the ceramic material; building up the coated area with -afmetal to form a 'tension band there- -aboutjshping the outer surface of said metallic band to the precalculated dimensions, expanding the inner diameter oi said jacket, contractin said band so that compressive 'stresses are set up in the ceramic material and the seal between the band and ceramic is tightened. and inserting the insultor into the plug jacket, whereby a gastight seal will be formed between the sui-!sees ol the metaliic shell and the surface of said insulator.

2, The method o! uniting paris oi dissimilar materials, wherein part of one material is to beinsertedintoasocketiormedintheother material which includes, the steps oi' platin: one ot said materials with a thin eoaflng o! metal.

building up said metal to' a desired thickness,.to form a tension band .about said one of said materials, shapingsaid bandto a precalculated diwhich is-to be inserted into said socket, whereby v the'binding action ofwsaid tension band will. be increased either' by the heating or cooling process to subject one of said parts to compressive stress, and finally inserting saidone part of one material into 'the 'socketformed into the other material so that a gas-tight seal is formed at the juncton of the two parts.

3.'In the manufacture of a spark plug the method of securing the ceramic insulator to the outer metallic jacket, which comprises the steps of forming a tensioning band around the ceramic insulator, heating the outer jacket to expand same increasing its inner diameter, cooling the ceramic insulator and tensioning band to contract the band, whereby its tensional stresses will be increased and compressive stresses will be set up in the insulator, and inserting the cooled ceramic insulator and band into the heated expanded jacket so that a gas-tight seal will be formed at the union of their two surfaces.

4. In the manufacture of a spark plug, the method of mounting and scaling the ceramic insulator to the outer metallic jacket and of mounting and scaling the electrode rod to the longitudinal bore formed in said insulator which comprises the steps of, plating the inner and outer surface of said ceramic insulator with a thin metallic coat formed in intimate contact therewith, building up the coated areas with a suitable metal so as to form metallic bands of a desired thickness, heating said outer jacket to expand same increasing its inner diameter, and cooling said ceramic insulator and bands so that the outer band will bind the insulator to increase the tightness of the seal and subject said ceramic to compressive stresses, inserting said insulator into said jacket, heating the insulator and jacket to expand the inner metallic band and prevent the hollow insulator from being subjected to compressive 'stresses caused by the shrinking of the 'outer metallic jacket prior` to the insertion of said electrode, and to cause the inner band to expand to balance out the compressive forces of said insulator and increase the tightness of said inner seal, cooling said electrode rod, and inouter band to expand same, cooling said insulator to contract same, inserting-the insulator into said outer band, heating the outer band and insulator to a uniform temperature to expand the insulator increasing the diameter of said longitudinal bore and setting up compressive stresses therein, cooling the inner band to contract same and insert-ing the innerband into the expanded longitudinal bore formedin the insulator, whereby the contracted outer band and expanded inner band will exert balanced radial forces on the inner and outer surface of the insulator, setting up compressive stresses only therein.

7. The method of sealing the insulator and bands as constructed in accordance with the method' set forth in claim-6, to a metallic jacket which comprises cooling the insulator and bands and heating the metallic jacket, and inserting the insulator and bands into the metallic jacket so that a seal is formed at the union of the outer band and jacket.

8. In a spark plug, the combination of an outer metallic jacket, a hollow cylindrical insulator positioned within said jacket and formed of a material capable of withstanding sever compressive stresses only, said insulator being provided with a longitudinal bore extending therethrough, a central electrode positioned within the hollow bore, means comprising an expanded and shrunk fit between the inner surface of said metallic jacket and said insulator and between the inner surface of said insulator and said electrode for sealing the'insulator to said jacket and to said electrode whereby said insulator will be subjected to substantially balanced inner and outer radial forces so that compressive stresses alone will be set up therein.

9. The invention as definedby claim 8, wherein said last named means comprises a contracted outer metallic band and an expanded inner metalserting said rod into the bore formed in said insulator.

5. In the manufacture of a spark plug, the method of uniting parts including an electrode,

- cerarnic insulator and metallic jacket to form a gas-tight seal therebetween which consists in tightly securing a band of soft metal around the central electrode, placing a band of soft metal around the inner surface of said metallic jacket.

so that a tight seal is formed between the band and jacket, heating the jacket and inner band to expand its inner diameter, cooling the ceramic insulator which has a longitudinal bore formed therein, inserting the insulator into the heated jacket, heating the insulator and metallic jacket to a uniformly high temperature so as to expand the insulator increasing the diameter of said longitudina bore and setting up compressivestresses therein, cooling the central electrode and band and inserting it into the longltudinal bore, whereby the insulator will be subjected to an inner and outer radial force when the assembly attains uniform temperature, increasing the seal lic band.

10. In the method of manufacture of a spark plug having a hollow porcelain insulator which includes the steps of applying a thin coating of metal to the inner and outer surface of said insulator, building up the coated area with a metal so as to form an inner and outer metallic band, and machining the inner and outer band to precalculated dimensions.

11. In an apparatus of the class described, an insulating material capable of withstanding severe compressive stresses only having a bore formed therein, an inner expanded metallic band formed in said bore, an outer compressive metallic 'band formed about said insulator radially opposite said inner band, and means for applying said inner and outer band to said insulator so that radial forces Will be transmitted from the band to the insulator, the stresses transmitted to said insulator by the binding action of said outer 'band being equal to or greater than the stresses transmitted to the insulator by the inner band so that compressive stresses only will be set up in said insulator.

12. In a spark plug, a body having a cylindrical bore forming a shell, a hollow cylindrical ceramic insulator bushing Secured within the bore of said shell so as to form a gas-tight seal therewith, a

and a plurality ot laminated mica washers positioned about said metallic electrode between the shoulder !ormed on the tip end oi' said electrode and said ceramic insulator, whereby a' long leskae path is provided !rom the tip end of the electrode to the metallic shell which is less susceptlble to lead iouling than the porcelain bushing through which`the electrode is sealed to said shell. c

13. A spark plug for internal combustion ensines eomprising a tubular ceramic insuator having a thin metalic mm i'ormed at diametrically opposed areas on the inner and outer suriace of said. insula'tor. respectively, said films being !used into the body o! said insulator, a. metallic Jacket surrounding said insulator--opposite the coatd area provided with said outer film, a metallic electrode disposed within the bore oi said tubular insulator, means comprising an inner expanded metanic ring tor sealing said electrode to said insulator at the inner surface area over which the metallic film has been applied, and means comprising an outer metallicishrunk ring !or sealing said metallic jacket to said insulator at the outer surface area over which the film has been applied whereby the metal oi the expanded and shrunk rinas will be pressed into inmate con- 'tact with the inner metallic film and electrode,

and outer metanic film and Jacket respectively to form a gastight seal.

1 4. A spark plug' !or an internal combustion 4 than is the ceramic insulator, means sealing said 4 electrode to said insulating member exerting an outwardly directed radial i'orce on said insulating member, and means scaling said insulating member to 'said jacket exerting an inwardly directed radisl force on said insulating member.

15. A spark plug tor internal combustion engines comprising a tubular ceramc insulatlng member, a metallic jacket surrounding said tubular member, a metsllic electrode disposed within the bore oi' said tubular member, means scaling said electrode to said member exerting an outwardly directed radial force on said member and means sealing said metallc jacket to said member exerting an inwardly directed radia iorce thereon, the inwardly directed rsdial force being equal to or greater than the outwardly directed radial force throughout the thei-mal range oi' operation of said spark plug whereby said ceramic insulating membr will be subjected to compressive stresses only.

16. The invention as denned in claim 15 wherein said first named means comprises an expanded inner metallic band posltioned between said electrode and inner'suri'ace of said insulating member, and said last named means comprises an outer shrunk band positioned between the inner surface oi said metallic jacket and the outer surface of said electrode.

17. In the method oi uniting parts o! dissimilar materials wherein one oi said materials is to be inserted into a socket tormed in the other material, said other material being capable of withstanding svere 'compressive stresses only, which includes the steps oi Iorming an outer metallic band about said socket, !orming an inner metallic 'band within said socket diametrically opposite said outer band, heating said other material and bands to expand the inner diameter oi said inner band and to set up compressive stresses in said other material, cooling said first named material to contrac't same, and inserting said first named material into the socket iormed by the inner band formed in said other material whereby the other materialbetween said inner and outer bands will be subjected to compressive stresses only throughout the temperature range o! operation.

- HENRY K. KING. 

